This invention relates to range finders and, more particularly, to a temperature tracking narrow band optical range finder which uses an improved GaAs avalanche detector to receive a reflected pulse from emitted photon energy directed toward a target.
An ultimate goal in the development of a laser self-detecting optical range finder is to use the same diode as an emitter and detector simultaneously. This is called a self-detecting laser diode. Such a diode would eliminate a detector, its support electronics, a spectral filter and a set of collecting optics. The state of development of this range finder appears to be premature for practical applications.
A self-detection mode of operation of a laser diode is shown and claimed in U.S. Pat. No. 3,937,575 issued Feb. 10, 1976 to Bateman. This method is only useful after a dead time of 100 to 200 nanoseconds, thereby restricting the range from 50 to 100 feet. For close ranging (5 to 10 feet), this method is impractical, even without dead time, due to electrical interference from high current pulses. Other arrangements are needed for pulse detection with acceptable sensitivity in the millivolt range.
In U.S. Pat. No. 4,142,200, issued Feb. 27, 1979 to Mizushima, et al., a structure is disclosed that produces a high avalanche gain near the junction interface due to a high electric field. In this patent such a structure is made of a silicon material, and no narrow band operation with high responsivity is claimed. An avalanche detector for receiving reflected signals from a target may be fabricated with silicon material using this patent teachings. However, it requires broadband filters to allow acceptance of a broad temperature tuning range (approximately 3 A/.degree.K.) of a GaAs laser diode emission. Since the spectral response of a silicon detector is relatively flat, compared to a laser diode spectral emission, the usual way to achieve narrow band detection is in conjunction with an optical interference filter. The need for this filter introduces a limitation to the silicon detectors, such as the compromise between bandwidth and the use of a wide field of view and fast optics forced by the angle effects. Another limitation is that a larger bandwidth is required to accommodate the laser temperature tuning.
A prior art GaAs photodiode may be used as a detector in a range finder device; however, such a photodiode response decreases rapidly for wavelengths longer than 860 nanometers, hence, these photodiodes are not suitable for detection of room temperature GaAs laser diode emissions with typical peak wavelengths of 880 nanometers. The reason is that the detector response depends on band-to-band absorption while the laser diode emissions usually involve a band-to-acceptor transition. Thus, the laser diode emissions have significantly longer wavelengths than the threshold wavelength of the prior art GaAs detector. This difference in wavelength is more remarkable when higher operating temperatures of the laser diode are considered.
Therefore, it is an object of this invention to provide a temperature tracking optical range finder to be used at close range to a target.
Another object of this invention is to provide an improved narrow band self-filtering GaAs detector to this range finder.
A further object of this invention is to provide such an optical range finder to maintain, over a wide temperature range, the narrow band optical ranging capability while sustaining high sensitivity.
Still another object of this invention is to provide such an optical range finder with sufficient sensitivity so as to be capable of detecting weak pulses, such as reflections from diffuse targets.